The present invention relates to a catalyst for producing a synthesis gas and to a process for the production of carbon monoxide.
A synthesis gas is a mixed gas containing hydrogen and carbon monoxide and is widely used as a raw material for the synthesis of ammonia, methanol, acetic acid, etc.
Such a synthesis gas may be produced by reaction of a hydrocarbon with steam and/or carbon dioxide in the presence of a catalyst. In the reaction, however, carbon deposition reactions occur as side reactions to cause carbon deposition which brings about a problem of catalyst poisoning.
The raw materials for the carbon deposition are a carbon-containing organic compound used as a raw material and CO produced in situ. The carbon deposition is accelerated as the partial pressures of these raw materials increase. Therefore, it is possible to reduce the amount of the carbon deposition by increasing the feed amount of steam and carbon dioxide while reducing the reaction pressure. In this case, however, it is necessary to excessively use steam and carbon dioxide in order to reduce the partial pressures of the carbon-containing organic compound and CO, so that several disadvantages are caused. For example, consumption of heat energy required for preheating steam and carbon dioxide increases. Further, costs for the separation of these gases from the product increase. Moreover, since a large reaction apparatus is required, the apparatus costs increase.
JP-A-5-208801 discloses a carbon dioxide-reforming catalyst containing a Group VIII metal supported on high purity, super-fine single crystal magnesium oxide. JP-A-6-279003 discloses a carbon dioxide-reforming catalyst containing a ruthenium compound supported on a carrier composed of a compound of at least one alkaline earth metal oxide and aluminum oxide. JP-A-9-168740 discloses a carbon dioxide-reforming catalyst containing rhodium supported on a carrier formed of a Group II-IV metal oxide or a lanthanoid metal oxide or a composite carrier composed of the above metal oxide and alumina. The reaction experiments using these catalysts are performed under ambient pressure. At a high pressure, which is industrially significant, these catalysts show a high carbon deposition activity and, hence, are not satisfactory as industrially applicable catalysts.
Carbon monoxide is widely utilized as a raw material for the synthesis of industrial products by, for example, hydroformylation. Carbon monoxide is generally produced by the reforming of methane with steam according to the reaction shown below to obtain a synthesis gas, from which carbon monoxide is subsequently separated:
CH4+H2O H2+CO. 
In this reaction, however, only 1 mole of carbon monoxide is produced per 3 mole of hydrogen. Thus, the process for the production of carbon monoxide is not efficient. In contrast, the reforming of methane with carbon dioxide proceeds as follows:
CH4+CO22H2+2CO. 
Thus, hydrogen and carbon monoxide are produced in an equimolar amount so that this process is more efficient than the reforming with steam. In this case, when carbon dioxide is added in excess relative to methane, carbon monoxide is produced from carbon dioxide and hydrogen by the following reverse shifting reaction:
CO2+H2CO+H2O, 
so that the concentration of carbon monoxide in the product gas further increases. Therefore, the reforming with carbon dioxide is effective in the production of carbon monoxide. However, the product gas obtained by this reaction has a composition in an equilibrium which favors the carbon deposition, so that the catalyst used for this reaction causes considerable deactivation of the catalyst.
The objects of the present invention are:
(1) to provide a catalyst for use in a process for the production of a synthesis gas by reaction of a carbon-containing organic compound with steam and/or carbon dioxide, which catalyst has suppressed carbon deposition activity;
(2) to provide a catalyst for use in a process for the production of a synthesis gas by reaction of a carbon-containing organic compound with oxygen, which catalyst has suppressed carbon deposition activity; and
(3) to provide a process which includes a step of reacting a carbon-containing organic compound with carbon dioxide to produce a synthesis gas, and a step of concentrating carbon monoxide in the thus obtained synthesis gas and which can produce carbon monoxide in an economically favorable manner by using a catalyst having suppressed carbon deposition activity in the synthesis gas producing step.
Other objects of the present invention will be understood from the following description of the specification.
The present inventors have made an intensive study to accomplish the above-described objects and, as a result, have completed the present invention.
In accordance with the present invention there is provided a catalyst for producing a synthesis gas comprising a carrier formed of a metal oxide and at least one catalytic metal selected from rhodium, ruthenium, iridium, palladium and platinum and supported on said carrier, characterized in that said catalyst has a specific surface area of 25 m2/g or less, in that the electronegativity of the metal ion of said carrier metal oxide is 13.0 or less and in that the amount of said supported catalytic metal is 0.0005-0.1 mole %, in terms of a metal, based on said carrier metal oxide.
The present invention also provides a process for producing carbon monoxide, which comprises a step of reacting a carbon-containing organic compound with carbon dioxide at an elevated temperature in a pressurized condition in the presence of a catalyst to produce a synthesis gas, and a step of concentrating carbon monoxide in the thus obtained synthesis gas, said process being characterized in that said catalyst comprises a carrier formed of a metal oxide and at least one catalytic metal selected from rhodium, ruthenium, iridium, palladium and platinum and supported on said carrier, in that said catalyst has a specific surface area of 25 m2/g or less, in that the electronegativity of the metal ion of said carrier metal oxide is 13.0 or less and in that the amount of said catalytic metal is 0.0005-0.1 mole %, in terms of metal, based on said carrier metal oxide.
The catalyst of the present invention is used for the production of a synthesis gas using a carbon-containing organic compound as a raw material. In this case, the processes for producing a synthesis gas include various conventionally known processes, for example, (i) a process in which a carbon-containing organic compound is reacted with steam, (ii) a process in which a carbon-containing organic compound is reacted with carbon dioxide, (iii) a process in which a carbon-containing organic compound is reacted with a mixture of steam with carbon dioxide and (iv) a process in which a carbon-containing organic compound is reacted with oxygen.
The catalyst of the present invention contains at least one catalytic metal selected from rhodium (Rh), ruthenium (Ru), iridium (Ir), palladium (Pd) and platinum (Pt) supported on a carrier metal oxide having specific characteristics. In this case, the catalytic metal can be supported in the form of a metallic state or in the form of a metal compound such as an oxide.
The catalyst of the present invention is characterized in that the catalyst has activity required for converting a carbon-containing organic compound into a synthesis gas while exhibiting a function to significantly suppress side reactions of carbon deposition reactions.
The catalyst according to the present invention can significantly suppress the carbon deposition reactions is characterized in that:
(i) the electronegativity of the metal ion of the carrier metal oxide is 13.0 or less;
(ii) the catalyst has a specific surface area of 25 m2/g or less; and
(iii) the amount of the supported catalytic metal is 0.0005-0.1 mole % based on the carrier metal oxide. Such a catalyst having a considerably suppressed carbon deposition activity has been first found by the present inventors.
The metal oxide used as a carrier may be a single metal oxide or a mixed metal oxide. In the present invention, the electronegativity of the metal ion in the carrier metal oxide is 13 or less, preferably 12 or less, more preferably 10 or less. The lower limit is about 4. Thus, the electronegativity of the metal ion in the carrier metal oxide used in the present invention is 4-13, preferably 4-12. The electronegativity of the metal ion in the metal oxide in excess of 13 is not preferable, because carbon deposition occurs significantly.
The electronegativity of the metal ion in the metal oxide is defined by the following formula:
Xi=(1+2i)Xo 
wherein Xi: electronegativity of the metal ion
Xo: electronegativity of the metal
i: valence electron number.
When the metal oxide is a mixed metal oxide, an average electronegativity of the metal ions is used. The average value is a sum of the products of the electronegativity of each of the metal ions contained in the mixed metal oxide by the molar fraction of the corresponding metal oxide of the mixed metal oxide.
The electronegativity (Xo) of a metal is in accordance with Pauling. The electronegativity in accordance with Pauling is as shown in xe2x80x9cW. J. Moore Physical Chemistry, Vol. 1 translated by FUJISHIRO, Ryoichixe2x80x9d, 4th Edition, Tokyo Kagaku Dojin, p. 707 (1974), Table 15.4.
The electronegativity of metal ion in a metal oxide is described in detail in, for example, xe2x80x9cSyokubaikoza, vol. 2, p145 (1985) edited by Catalyst Society of Japanxe2x80x9d.
The metal oxides may include those containing one or at least two metals such as Mg, Ca, Ba, Zn, Al, Zr and La. Illustrative of such metal oxides are single metal oxides such as magnesia (MgO), calcium oxide (CaO), barium oxide (BaO), zinc oxide (ZnO), alumina (Al2O3), zirconia (ZrO2) and lanthanum oxide (La2O3), and mixed metal oxides such as MgO/CaO, MgO/BaO, MgO/ZnO, MgO/Al2O3, MgO/ZrO2, CaO/BaO, CaO/ZnO, CaO/Al2O3, CaO/ZrO2, BaO/ZnO, BaO/Al2O3, BaO/ZrO2, ZnO/Al2O3, ZnO/ZrO2, Al2O3/ZrO2, La2O3/MgO, La2O3/Al2O3 and La2O3/CaO.
The catalyst according to the present invention having a specific surface area of 25 m2/g or less may be obtained by calcining a carrier metal oxide before the support of a catalytic metal at 300-1,300xc2x0 C., preferably 650-1,200xc2x0 C. After the catalytic metal has been supported, the catalytic metal-supported carrier is further calcined at 600-1,300xc2x0 C., preferably 650-1,200xc2x0 C. It is also possible to obtain the catalyst by supporting a catalytic metal on a carrier metal oxide, followed by the calcination of the catalytic metal supporting product at 600-1,300xc2x0 C., preferably 650-1,200xc2x0 C. The upper limit of the calcination temperature is not specifically limited but is generally 1,500xc2x0 C. or less, preferably 1,300xc2x0 C. or less. In this case, the specific surface area of the catalyst or the carrier metal oxide can be controlled by the calcination temperature and calcination time.
The specific surface area of the catalyst or the carrier metal oxide used in the present invention is preferably 20 m2/g or less, more preferably 15 m2/g or less, most preferably 10 m2/g or less. The lower limit of the specific surface area is about 0.01 m2/g. By specifying the specific surface area of the catalyst or the carrier metal oxide in which the electronegativity of the metal ion is 13 or less in the above range, the carbon deposition activity of the catalyst can be significantly suppressed.
The amount of the catalytic metal supported on the carrier metal oxide is at least 0.0005 mole %, preferably at least 0.001 mole %, more preferably at least 0.002 mole %, in terms of metal, based on the carrier metal oxide. The upper limit is generally 0.1 mole %, preferably 0.09 mole %. In the present invention, the amount of metal supported is desirably in the range of 0.0005 -0.1 mole %, preferably 0.001-0.1 mole %.
In the catalyst of the present invention, the specific surface area of the catalyst is substantially the same as that of the carrier metal oxide. Thus, in the present specification, the term xe2x80x9cspecific surface area of a catalystxe2x80x9d is used as having the same meaning as xe2x80x9cspecific surface area of a carrier metal oxide thereofxe2x80x9d.
The term xe2x80x9cspecific surface areaxe2x80x9d referred to in the present specification in connection with a catalyst or a carrier metal oxide is as measured by the xe2x80x9cBET methodxe2x80x9d at a temperature of 15xc2x0 C. using a measuring device xe2x80x9cSA-100xe2x80x9d manufactured by Shibata Science Inc.
The catalyst according to the present invention has a small specific surface area and has an extremely small amount of a supported catalytic metal so that the carbon deposition activity thereof is considerably suppressed. Yet, the catalyst has satisfactory activity for converting a raw material carbon-containing organic compound into a synthesis gas.
The catalyst of the present invention may be prepared by conventional methods. One preferred method of preparing the catalyst of the present invention is an impregnation method. To prepare the catalyst of the present invention by the impregnation method, a catalyst metal salt or an aqueous solution thereof is added to and mixed with an aqueous dispersion containing a carrier metal oxide. The carrier metal oxide is then separated from the aqueous solution, followed by drying and calcination. A method (incipient-wetness method) is also effective in which a carrier metal oxide is added with a solution of a metal salt little by little in an amount corresponding to a pore volume to uniformly wet the surface of the carrier, followed by drying and calcination. In these methods, a water soluble salt is used as the catalyst metal salt. Such a water soluble salt may be a salt of an inorganic acid, such as a nitrate or a hydrochloride, or a salt of an organic acid, such as an acetate or an oxalate. Alternately, a metal acetylacetonate, etc. may be dissolved in an organic solvent such as acetone and the solution may be impregnated into the carrier metal oxide. The drying is performed at a temperature of 100-200xc2x0 C., preferably 100-150xc2x0 C. when the metal oxide is impregnated with an aqueous solution of a catalytic metal salt. When the impregnation is performed using an organic solvent, the drying is performed at a temperature higher by 50-100xc2x0 C. than the boiling point of the solvent. The calcination temperature and time are adequately selected according to the specific surface area of the carrier metal oxide or catalyst obtained (the specific surface area of the catalyst). Generally, a calcination temperature in the range of 500-1,100xc2x0 C. is used.
In the preparation of the catalyst of the present invention, the metal oxide used as a carrier may be a product obtained by calcining a commercially available metal oxide or a commercially available metal hydroxide. The purity of the metal oxide is at least 98% by weight, preferably at least 99% by weight. It is, however, undesirable that components which enhance carbon deposition activity or components which are decomposed under reducing conditions, such as metals, e.g. iron and nickel, and silicon dioxide (SiO2) . Such impurities in the metal oxide are desired to be not greater than 1% by weight, preferably not greater than 0.1% by weight.
The catalyst of the present invention may be used in various forms such as powdery, granular, spherical, columnar and cylindrical forms. The form may be appropriately selected according to the catalytic bed system used.
The production of a synthesis gas using the catalyst of the present invention may be performed by reacting a carbon-containing organic compound with steam and/or carbon dioxide (CO2) or by reacting a carbon-containing organic compound with oxygen in the presence of the catalyst. As the carbon-containing organic compound, a lower hydrocarbon such as methane, ethane, propane, butane or naphtha or a non-hydrocarbon compound such as methanol or dimethyl ether may be used. The use of methane is preferred. In the present invention, a natural gas (methane gas) containing carbon dioxide is advantageously used.
In the case of a method of reacting methane with carbon dioxide (CO2) (reforming with CO2), the reaction is as follows:
CH4+CO22H2+2CO xe2x80x83xe2x80x83(1) 
In the case of a method of reacting methane with steam (reforming with steam), the reaction is as follows:
CH4+H2O 3H2+CO xe2x80x83xe2x80x83(2) 
In the reforming with CO2, the reaction temperature is 500-1,200xc2x0 C., preferably 600-1,000xc2x0 C. and the reaction pressure is an elevated pressure of 5-40 kg/cm2G, preferably 5-30 kg/cm2G. When the reaction is performed with a packed bed system, the gas space velocity (GHSV) is 1,000-10,000 hrxe2x88x921, preferably 2,000-8,000 hrxe2x88x921. The amount of CO2 relative to the raw material carbon-containing organic compound is 20-0.5 mole, preferably 10-1 mole, per mole of carbon of the raw material compound.
In the reforming with steam, the reaction temperature is 600-1,200xc2x0 C., preferably 600-1,000xc2x0 C. and the reaction pressure is an elevated pressure of 1-40 kg/cm2G, preferably 5-30 kg/cm2G. When the reaction is performed with a packed bed system, the gas space velocity (GHSV) is 1,000-10,000 hrxe2x88x921, preferably 2,000-8,000 hrxe2x88x921. The amount of steam relative to the raw material carbon-containing organic compound is 0.5-5 moles, preferably 1-2 moles, more preferably 1-1.5 moles, per mole of carbon of the raw material compound.
In the reforming with steam according to the present invention, it is possible to produce a synthesis gas in an industrially favorable manner while suppressing the carbon deposition, even when the amount of steam (H2O) is maintained 2 moles or less per mole of carbon of the raw material compound. In view of the fact that 2-5 moles of steam per mole of carbon in the raw material compound is required in the conventional method, the catalyst of the present invention, which can permit the reforming reaction to smoothly proceed with an amount of steam of 2 moles or less, has a great industrial merit.
The catalyst of the present invention is favorably used as a catalyst for reacting a carbon-containing organic compound with a mixture of steam and CO2. In this case, the mixing proportion of steam and CO2 is not specifically limited but is generally such as to provide a H2O/CO2 molar ratio of 0.1-10.
When a carbon-containing organic compound is reacted with oxygen using the catalyst of the present invention, the carbon-containing organic compound may be such a hydrocarbon or non-hydrocarbon organic compound as described previously and is preferably methane. As the source of oxygen, there may be used oxygen, air or oxygen-rich air. In the present invention a natural gas (methane gas) containing carbon dioxide is advantageously used as a reaction raw material.
In the case of the reaction of methane with oxygen, the reaction is as shown below:
CH4+{fraction (1/20)}2 CO+2H2 xe2x80x83xe2x80x83(3) 
In partial oxidation of the carbon-containing organic compound, the reaction temperature is 500-1,500xc2x0 C., preferably 700-1,200xc2x0 C. and the reaction pressure is an elevated pressure of 5-50 kg/cm2G, preferably 10-40 kg/cm2G. When the reaction is performed with a packed bed system, the gas space velocity (GHSV) is 1,000-50,000 hrxe2x88x921, preferably 2,000-20,000 hrxe2x88x921. The amount of oxygen relative to the raw material carbon-containing organic compound is such as provide a molar ratio of carbon of the raw material carbon-containing organic compound to oxygen molecules C/O2 of 4-0.1 mole, preferably 2-0.5 mole. Since the partial oxidation method is a greatly exothermic reaction, it is possible to adopt a reaction system of autothermic system while adding steam and carbon dioxide to the raw material.
The above-described various reactions using the catalyst of the present invention may be carried out with various catalyst systems such as a packed bed system, a fluidized bed system, a suspension bed system and a moving bed system.
A process for the production of carbon monoxide according to the present invention includes, as a first step, a synthesis gas producing step. The first step is carried out by reacting a carbon-containing organic compound with carbon dioxide in the presence of a catalyst. In this case, the previously described catalyst is used as the synthesis gas production catalyst.
In the reaction of the carbon-containing organic compound with carbon dioxide (reforming with CO2), the reaction temperature is 500-1,200xc2x0 C., preferably 600-1,000xc2x0 C. and the reaction pressure is an elevated pressure of 1-40 kg/cm2G, preferably 5-30 kg/cm2G. When the reaction is performed with a packed bed system, the gas space velocity (GHSV) is 1,000-10,000 hrxe2x88x921, preferably 2,000-8,000 hrxe2x88x921. The amount of carbon dioxide relative to the raw material carbon-containing organic compound is 1-10 moles, preferably 1-5 moles, more preferably 1-3 moles, per mole of carbon of the raw material compound.
In the reforming with CO2 according to the present invention, a synthesis gas can be produced in an industrially advantageous manner while preventing carbon deposition, even when the amount of CO2 is maintained no more than 3 moles per mole of carbon of the raw material compound.
The above-described reforming with CO2 may be carried out with various catalyst systems such as a packed bed system, a fluidized bed system, a suspension bed system and a moving bed system and is preferably performed using a packed bed system.
As a result of the above-described reforming with CO2, a synthesis gas containing hydrogen and carbon monoxide is obtained. When methane is used as a raw material, the synthesis gas has, for example, a composition containing 10-30 vol % of H2, 35-45 vol % of CO, 5-40 vol % of unreacted CO2, 0-30 vol % of unreacted CH4 and 5-20 vol % of H2O.
In the second step of the present invention, the thus obtained synthesis gas is used as a raw material and carbon monoxide (CO) is concentrated therefrom. The CO concentration may be carried out by a customarily employed CO concentration method such as a cryogenic separation and a absorption method using an aqueous copper salt solution as an absorbent.